1. Field of the Invention
This invention relates generally to the slip casting of ceramic articles, i.e., the process of forming ceramic articles by pouring a ceramic slip (e.g., a mixture of finely-divided fused silica and water having the consistency of cream) into a plaster of paris mold, followed by firing to produce the finished article. In particular, the invention relates to an improved ceramic slip casting method which employs a flask molding material for non-ferrous metals, viz., a calcium sulfate-bonded investment containing calcinated silica and glass fibers, as a shell mold instead of plaster of paris.
2. Description of the Related Art
A primary concern in modern fluid dynamics research is the experimental verification of computational aerothermodynamic codes. This research requires high precision and detail in the test model employed. Ceramic materials are used for these models because of their low heat conductivity and their survivability at high temperatures. To fabricate such models, slip casting techniques were developed to provide net-form, precision casting capability for high-purity ceramic materials in aqueous solutions.
Casting of ceramic models was desired over machining, since a cast model can be easily and inexpensively replicated. However, because of the brittleness of ceramic models, many are broken in the rough testing environment, before, during, and after wind-tunnel runs. The art of slip casting was developed for this reason. Slip casting was found to provide superior results over other ceramic casting techniques, such as hydraulic casting. The plaster mold draws out moisture through the mold interface, pulling or sucking the slip material into the desired shape. This removes air gaps, and the slip has minimum shrinkage from the mold during setup. In fact, a plaster mold will grow when cured, and many times this will completely compensate for shrinkage in the slip material.
In these previous slip casting techniques, block, or flask molds made of plaster-of-paris were used to draw liquid from the slip material. Upon setting, parts were removed from the flask mold and cured in a kiln at high temperatures. Casting detail was usually limited with this technique--detailed parts were frequently damaged upon separation from the flask mold, as the molded parts are extremely delicate in the uncured state, and the flask mold is inflexible. Ceramic surfaces were also marred by "parting lines" caused by mold separation. This adversely affected the aerodynamic surface quality of the model as well. (Parting lines are invariably necessary on or near the leading edges of wings, nosetips, and fins for mold separation. These areas are also critical for flow boundary layer control.)
Parting agents used in the casting process also affected surface quality. These agents eventually soaked into the mold, the model, or flaked off when releasing the cast model. Different materials were tried, such as oils, paraffin, and even an algae. The algae released best, but some of it remained on the model and imparted an uneven texture and discoloration on the model surface when cured. Unavailable in the prior art was a technique providing for the casting of complete, detailed, precision models without parting lines or the use of parting agents.
U.S. Pat. No. 4,659,526 is directed to a forming method and a mold wherein a slip is cast into the mold, and the mold is removed after hardening of the slip. The mold is patterned by the use of a water-soluble binder, and it absorbs a water content in the slip to soften itself and become easy for removal. This mold is made of refractories other than calcium, and there is no mention of employing such material in a shell mold application.
U.S. Pat. No. 4,556,528 is directed to a method and apparatus for the casting of fragile complex shapes. However, the mold material employed is a wax, which cannot be used to draw liquid from a ceramic slip.
U.S. Pat. No. 4,338,272 is directed to a slip casting system which employs a nonplaster, viz., ceramic powder, for the mold. Although this system provides for the easy removal of large areas of the molded article, it will not work well with detailed shapes, as the mold is rigid.
U.S. Pat. No. 2,494,403 is directed to a molding composition for ceramic slip casting which is a high performance plaster type material. However, this mold material hardens when wet (no detail may be cast) and weakens when hot (no wax pattern may be made).
Accordingly, it is a primary object of the present invention to provide what is not available in the art, viz., an improved ceramic slip casting technique which affords the casting of complete, detailed, precision articles without parting lines or the use of parting agents, the articles being especially suitable as wind tunnel models for use in the experimental verification of computational aerothermodynamic codes.